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</p><h2 id="operators"><a href="#operators" aria-hidden="true" class="header-anchor">#</a> Operators</h2><h3 id="distinct"><a href="#distinct" aria-hidden="true" class="header-anchor">#</a> distinct</h3><p>如果会下 SQL 指令的应该都对 distinct 不陌生，它能帮我们把相同值的资料滤掉只留一笔，RxJS 里的 distinct 也是相同的作用，让我们直接来看示例</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token keyword">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">zip</span><span class="token punctuation">(</span>Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token function">interval</span><span class="token punctuation">(</span><span class="token number">300</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source<span class="token punctuation">.</span><span class="token function">distinct</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

example<span class="token punctuation">.</span><span class="token function">subscribe</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
    next<span class="token punctuation">:</span> <span class="token punctuation">(</span>value<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    error<span class="token punctuation">:</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Error: '</span> <span class="token operator">+</span> err<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    complete<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'complete'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
<span class="token comment">// c</span>
<span class="token comment">// complete</span>

</code></pre><p><a href="https://jsbin.com/dipabe/2/edit?js,console" target="_blank" rel="noopener noreferrer">JSBin</a> | <a href="https://jsfiddle.net/3pfs88g8/" target="_blank" rel="noopener noreferrer">JSFiddle</a></p><p>如果用 Marble Diagram 表示如下</p><pre class="language-text"><code>source : --a--b--c--a--b|
            distinct()
example: --a--b--c------|

</code></pre><p>从上面的示例可以看得出来，当我们用 distinct 后，只要有重复出现的值就会被过滤掉。</p><p>另外我们可以传入一个 selector callback function，这个 callback function 会传入一个接收到的元素，并回传我们真正希望比对的值，举例如下</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token keyword">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">{</span> value<span class="token punctuation">:</span> <span class="token string">'a'</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> value<span class="token punctuation">:</span> <span class="token string">'b'</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> value<span class="token punctuation">:</span> <span class="token string">'c'</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> value<span class="token punctuation">:</span> <span class="token string">'a'</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> value<span class="token punctuation">:</span> <span class="token string">'c'</span> <span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">zip</span><span class="token punctuation">(</span>Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token function">interval</span><span class="token punctuation">(</span><span class="token number">300</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source<span class="token punctuation">.</span><span class="token function">distinct</span><span class="token punctuation">(</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> x<span class="token punctuation">.</span>value
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

example<span class="token punctuation">.</span><span class="token function">subscribe</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
    next<span class="token punctuation">:</span> <span class="token punctuation">(</span>value<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    error<span class="token punctuation">:</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Error: '</span> <span class="token operator">+</span> err<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    complete<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'complete'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// {value: &quot;a&quot;}</span>
<span class="token comment">// {value: &quot;b&quot;}</span>
<span class="token comment">// {value: &quot;c&quot;}</span>
<span class="token comment">// complete</span>

</code></pre><p><a href="https://jsbin.com/dipabe/3/edit?js,console" target="_blank" rel="noopener noreferrer">JSBin</a> | <a href="https://jsfiddle.net/3pfs88g8/2/" target="_blank" rel="noopener noreferrer">JSFiddle</a></p><p>这里可以看到，因为 source 送出的都是实例，而 js 事件的比对是比对内存位置，所以在这个例子中这些实例永远不会相等，但实际上我们想比对的是实例中的 value，这时我们就可以传入 selector callback，来选择我们要比对的值。</p><blockquote><p>distinct 传入的 callback 在 RxJS 5 几个 bate 版本中有过很多改变，现在网路上很多文章跟教学都是过时的，请读者务必小心！</p></blockquote><p>实际上 <code>distinct()</code> 会在背地里建立一个 Set，当接收到元素时会先去判断 Set 内是否有相同的值，如果有就不送出，如果没有则存到 Set 并送出。所以记得尽量不要直接把 distinct 用在一个无限的 observable 里，这样很可能会让 Set 越来越大，建议大家可以放第二个参数 flushes，或用 distinctUntilChanged</p><blockquote><p>这里指的 Set 其实是 RxJS 自己实现的，跟 ES6 原生的 Set 行为也都一致，只是因为 ES6 的 Set 支持程度还并不理想，所以这里是直接用 JS 实现。</p></blockquote><p>distinct 可以传入第二个参数 flushes observable 用来清除暂存的资料，示例如下</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token keyword">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">zip</span><span class="token punctuation">(</span>Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token function">interval</span><span class="token punctuation">(</span><span class="token number">300</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> flushes <span class="token operator">=</span> Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token function">interval</span><span class="token punctuation">(</span><span class="token number">1300</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source<span class="token punctuation">.</span><span class="token function">distinct</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> flushes<span class="token punctuation">)</span><span class="token punctuation">;</span>

example<span class="token punctuation">.</span><span class="token function">subscribe</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
    next<span class="token punctuation">:</span> <span class="token punctuation">(</span>value<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    error<span class="token punctuation">:</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Error: '</span> <span class="token operator">+</span> err<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    complete<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'complete'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
<span class="token comment">// c</span>
<span class="token comment">// c</span>
<span class="token comment">// complete</span>

</code></pre><p><a href="https://jsbin.com/dipabe/4/edit?js,console" target="_blank" rel="noopener noreferrer">JSBin</a> | <a href="https://jsfiddle.net/3pfs88g8/3/" target="_blank" rel="noopener noreferrer">JSFiddle</a></p><p>这里我们用 Marble Diagram 比较好表示</p><pre class="language-text"><code>source : --a--b--c--a--c|
flushes: ------------0---...
        distinct(null, flushes);
example: --a--b--c-----c|

</code></pre><p>其实 flushes observable 就是在送出元素时，会把 distinct 的暂存清空，所以之后的暂存就会从头来过，这样就不用担心暂存的 Set 越来愈大的问题，但其实我们平常不太会用这样的方式来处理，通常会用另一个方法 distinctUntilChanged。</p><h3 id="distinctuntilchanged"><a href="#distinctuntilchanged" aria-hidden="true" class="header-anchor">#</a> distinctUntilChanged</h3><p>distinctUntilChanged 跟 distinct 一样会把相同的元素过滤掉，但 distinctUntilChanged 只会跟最后一次送出的元素比较，不会每个都比，举例如下</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token keyword">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">zip</span><span class="token punctuation">(</span>Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token function">interval</span><span class="token punctuation">(</span><span class="token number">300</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>x<span class="token punctuation">,</span> y<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source<span class="token punctuation">.</span><span class="token function">distinctUntilChanged</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

example<span class="token punctuation">.</span><span class="token function">subscribe</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
    next<span class="token punctuation">:</span> <span class="token punctuation">(</span>value<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    error<span class="token punctuation">:</span> <span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Error: '</span> <span class="token operator">+</span> err<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
    complete<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'complete'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
<span class="token comment">// c</span>
<span class="token comment">// b</span>
<span class="token comment">// complete</span>

</code></pre><p><a href="https://jsbin.com/dipabe/6/edit?js,console" target="_blank" rel="noopener noreferrer">JSBin</a> | <a href="https://jsfiddle.net/3pfs88g8/4/" target="_blank" rel="noopener noreferrer">JSFiddle</a></p><p>这里 distinctUntilChanged 只会暂存一个元素，并在收到元素时跟暂存的元素比对，如果一样就不送出，如果不一样就把暂存的元素换成刚接收到的新元素并送出。</p><pre class="language-text"><code>source : --a--b--c--c--b|
            distinctUntilChanged()
example: --a--b--c-----b|

</code></pre><p>从 Marble Diagram 中可以看到，第二个 c 送出时刚好上一个就是 c 所以就被滤掉了，但最后一个 b 则跟上一个不同所以没被滤掉。</p><p>distinctUntilChanged 是比较常在开发中上使用的，最常见的状况是我们在做多方同步时。当我们有多个 Client，且每个 Client 有着各自的状态，Server 会再一个 Client 需要变动时通知所有 Client 更新，但可能某些 Client 接收到新的状态其实跟上一次收到的是相同的，这时我们就可用 distinctUntilChanged 方法只处理跟最后一次不相同的讯息，像是多方通话、多装置的资讯同步都会有类似的情境。</p><h2 id="今日小结"><a href="#今日小结" aria-hidden="true" class="header-anchor">#</a> 今日小结</h2><p>今天讲了两个 distinct 方法，这两个方法平常可能用不太到，但在需求复杂的应用里是不可或缺的好方法，尤其要处理非常多人即时同步的情境下，这会是非常好用的方法，不知道读者们今天有没有收获呢？ 如果有任何问题，欢迎在下方留言给我，感谢！</p></div></div></div><div class="v-card__actions"><div class="flex xs12"><a href="/BLOG/tags/RXJS"><span tabindex="0" class="v-chip capitalize chip-tag v-chip--label v-chip--small"><span class="v-chip__content">RXJS</span></span></a></div></div></article></div><div class="flex text-xs-left xs6"><a href="/BLOG/posts/rxjs16.html" class="post-nav v-btn v-btn--flat v-btn--router"><div class="v-btn__content"><div class="grey--text"><i class="fa mr-1 fa-chevron-left"></i>Prev</div><div class="title mt-1 primary--text hidden-xs-only">16 Observable Operators - catch, retry, retryWhen, repeat</div></div></a></div><div class="flex text-xs-right xs6"><a href="/BLOG/posts/rxjs14.html" class="post-nav v-btn v-btn--flat v-btn--router"><div class="v-btn__content"><div class="grey--text">Next
          <i class="fa ml-1 fa-chevron-right"></i></div><div class="title mt-1 primary--text hidden-xs-only">14 Observable Operator - throttle, debounce</div></div></a></div><div class="flex mt-3 xs12"><div class="v-card" style="height:undefined;"><div class="v-card__title"><span class="headline">Comment</span></div></div></div></div></div><footer class="v-footer blog-footer darken-1 mt-3 theme--dark" style="height:auto;"><div class="primary--text text--lighten-4 text-xs-center py-3 v-card v-card--flat v-card--tile primary" style="height:undefined;"><div class="v-card__text pb-0">博客内容遵循 <a rel="license noopener noreferrer" href="https://creativecommons.org/licenses/by-nc-sa/4.0/deed.zh" target="_blank">知识共享 署名 - 非商业性 - 相同方式共享 4.0 国际协议</a></div><div class="v-card__text pt-0 mt-1"><span>訾绍飞 © 2015 - 2018</span><span><!---->
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